1. Field of the Invention
The present invention relates to a light emitting element which emits light by feeding current and a light emitting device using the light emitting element.
2. Description of the Related Art
A light emitting device using a light emitting element having a layer including an organic material between a pair of electrodes which emits light by feeding current between the electrodes has been developed. Such a light emitting device is advantageous to be a thin shape and lightweight compared with other display devices which are referred to as thin display devices, has favorable visibility because of self-light emission, and has high response speed. Therefore, the light emitting devices are actively developed as display devices for the next generation, and at the present day, part of the light emitting devices is in practical use.
In such a light emitting element, holes are injected from an electrode which functions as an anode and electrons are injected from an electrode which functions as a cathode in a layer including an organic material. Then, a light emitting material in the layer including an organic material is excited by recombining holes and electrons, and light corresponding to the difference in energy between a ground state and an excited state is emitted when the light emitting material returns to a ground state.
Such a light emitting device which is advantageous to be a thin shape and lightweight is especially suitable for application of a mobile device. The lower power consumption of a light emitting device mounted to a mobile device in which there is a limitation in battery, the better, and power saving is always requested. In addition, a requirement of reducing energy consumption is more and more increased in association with an environmental problem, an energy problem, and the like in a television, a display, and the like in addition to a mobile device.
In particular, a television, a display, or the like with beauty in an image is a deserved requirement for a consumer now. One of the factors which affect beauty in an image is color reproductivity. In a case of a full-color display device, all colors are reproduced using three colors of red, green, and blue, or four colors of red, green, blue, and white. A bright display can be obtained by using a material which emits light with a wavelength having higher color purity; however, there are only a few materials with both high color purity and high reliability in the present situation.
The enhancement of external extraction efficiency of light emitted from a light emitting element is also effective for reducing power consumption. Patent Document 1 (Patent Document 1: Japanese Patent Laid-Open No. 2000-323277) mentions that, to enhance external extraction efficiency of light, each layer of an organic compound material layer except for a light emitting layer is set to have a different thickness corresponding to luminescent color and a reflection interference phenomenon is utilized, and accordingly, extraction efficiency of each color is enhanced. Patent Document 2 (Patent Document 2: Japanese Patent Laid-Open No. 2003-142277) mentions that each transparent electrode is set to have a different thickness corresponding to luminescent color and an interference phenomenon of reflected light is utilized, and accordingly, extraction efficiency of each color is enhanced.
However, in the structure in Patent Document 1, voltage and luminance efficiency is reduced in a case where a thickness of each layer of the organic compound material layer is set in consideration of a reflection interference phenomenon, while the thickness of each layer of the organic compound material layer is necessary to be set so that the function thereof is shown essentially. In addition, in the structure in Patent Document 2, the reduction in voltage and luminance efficiency is small. However, in order to form a transparent electrode corresponding to each color, the number of processes is increased because one of the electrodes of the light emitting element is formed so as to form a transparent electrode corresponding to each color, and productivity is very low. Thus, it is an object of the present invention to improve color purity of a light emitting element without reduction in voltage and luminance efficiency and current and luminance efficiency.
As described above, it is favorable as power consumption is lower, and the present situation is far from a situation that low power consumption is sufficiently attained. Therefore, power consumption is desired to be further lowered. Thus, it is another object of the present invention to provide a light emitting element and a light emitting device capable of lowering power consumption.